Grass with new genes sucks up pollution from explosives

Bacteria that eats the toxic chemical RDX was introduced into
two species of grass by engineers at the University of Washington. The
best-performing strains of bacteria removed all the RDX from soil in
just two weeks. Photo by Dennis Wise, University of Washington

A new transgenic grass species can neutralize and eradicate RDX, a toxic compound widely used in explosives since World War II.

On military live-fire training ranges, troops
practice firing artillery shells, drop bombs on old tanks or derelict
buildings, and test the capacity of new weapons. But those explosives
and munitions leave behind toxic compounds that have contaminated
millions of acres of U.S. military bases -- with an estimated cleanup
bill ranging between $16 billion and $165 billion.

As reported in Plant Biotechnology Journal,
engineers introduced two genes from bacteria that learned to eat RDX and
break it down into harmless components in two perennial grass species:
switchgrass (Panicum virgatum) and creeping bentgrass (Agrostis
stolonifera). The best-performing strains removed all the RDX from a
simulated soil in which they were grown within less than two weeks, and
they retained none of the toxic chemical in their leaves or stems.

It is the first reported demonstration of
genetically transforming grasses to supercharge their ability to remove
contamination from the environment. Grasses are hearty, fast-growing,
low-maintenance plants that offer practical advantages over other
species in real-world cleanup situations.

"This is a sustainable and affordable way to
remove and destroy pollutants on these training ranges," says senior
author Stuart Strand, professor of civil and environmental engineering
at the University of Washington whose lab focuses on taking genes from
microorganisms and animals that are able to degrade toxic compounds and
engineering them into useful plants.

"The grasses could be planted on the training
ranges, grow on their own and require little to no maintenance. When a
toxic particle from the munitions lands in a target area, their roots
would take up the RDX and degrade it before it can reach groundwater,"
Strand says.

RDX is an organic compound that forms the base
for many common military explosives, which can linger in the
environment in unexploded or partially exploded munitions. In large
enough doses, it has been shown to cause seizures and organ damage, and
it's currently listed by the Agency for Toxic Substances & Disease
Registry as a potential human carcinogen.

Unlike other toxic explosives constituents
such as TNT -- which binds to soils and tends to stay put -- RDX
dissolves easily in water and is more prone to spread contamination
beyond the limits of a military range, manufacturing facility, or
battleground.

"Particles get scattered around and then it
rains," Strand says. "Then RDX dissolves in the rainwater as it moves
down through the soil and winds up in groundwater. And, in some cases,
it flows off base and winds up in drinking water wells."

Wild grass species do remove RDX contamination
from the soil when they suck water up through their roots, but they
don't significantly degrade it. So when the grasses die, the toxic
chemical is re-introduced into the landscape.

Coauthors Neil Bruce and Liz Rylott,
biotechnology professor and research scientist, respectively, at the
University of York and colleagues had previously isolated enzymes found
in bacteria that evolved to use the nitrogen found in RDX as a food
source. That digestion process has the added benefit of degrading the
toxic RDX compound into harmless constituents.

The bacteria themselves aren't an ideal
cleanup tool because they require other food sources that aren't always
present on military training ranges. So Bruce and Rylott tried inserting
the bacterial genes into plant species commonly used in laboratory
settings. Those experiments proved that the new plant strains were able
to remove RDX contamination much more successfully than their wild
counterparts.

"Considering the worldwide scale of explosives
contamination, plants are the only low cost, sustainable solution to
cleaning up these polluted sites," says Bruce.

The team of civil and environmental engineers
at the University of Washington spent eight years working to express the
same genes in plant species that could stand up to real-world use. They
needed a hearty perennial species that grows back year after year and
that has strong root systems that can bounce back after fires.

Grasses fit that bill, but they are more
difficult to manipulate genetically. In particular, the engineers had to
build into their gene constructs robust monocot "promoters"—or regions
of DNA that cause a particular gene to be expressed—for the process to
work in grass species.

"For cleaning up contaminated soils, grasses
work best, but they're definitely not as easy to transform, especially
since flexible systems to express multiple genes in grasses have not
been used before," says first author and acting instructor Long Zhang.

The research team also found another
unexpected side benefit: because the genetically modified grasses use
RDX as a nitrogen source, they actually grow faster than wild grass
species.

Next steps for the research team include
limited field trials on a military training range to test how the
strains perform under different conditions. Wider use would require USDA
approval to ensure that the genetic modifications pose no threat to
wild grass species.

"I think it would be ecologically acceptable
because the genes we've introduced degrade real pollutants in the
environment and cause no harm," Strand says. "From my perspective, this
is a useful technology that's beneficial to the environment and has the
potential to remove dangerous legacy contamination from decades of
military activity."